The invention has application in multistage pumps suitable for pumping a range of fluids, both liquids and gases, but also offers particular advantages in relation to axial pumps for use in pumping multiphase fluids as may be encountered in oil and gas exploration and production. Both the general and multiphase applications of the invention are described herein.
In existing multistage fluid pumps and compressors, whether of the centrifugal, mixed flow or axial flow type, an increase in fluid pressure is achieved in each stage by: an impeller, which imparts both whirl to the fluid and increases its pressure; and a diffuser or volute, which reduces the absolute velocity of the fluid and increases the fluid pressure further by the partial conversion of fluid velocity energy into pressure energy. In general, an objective in the design of these machines is that at the flow rate at which the hydraulic efficiency is a maximum that is, the design duty flow, a substantial amount of fluid diffusion takes place in the volute or bladed stators.
In order to achieve relatively high stage pressures it is generally necessary to employ centrifugal or mixed flow pumps. It is among the objects of one aspect of the present invention to provide a multistage axial flow pump which will provide a performance comparable with, or better than, a multistage centrifugal or mixed flow pump, at a lower manufacturing cost.
In many oil fields the fluid which is extracted from a hydrocarbon reservoir is a mixture of gas and liquid phases. During the pumping of such fluid, particularly at lower pressures, the gas phase tends to separate from the liquid phase, this problem being particularly acute within pump stages. In a conventional axial pump the gas phase tends to accumulate around the axis of the pump and to flow back along the pump axis.
Such conventional pumps typically comprise a cylindrical casing within which is mounted a rotatable shaft. An axial flow impeller, which may have a cylindrical or a conical hub, is mounted on the shaft directly upstream of a stationary diffuser. The impeller adds energy to the fluid while the diffuser reduces the absolute velocity of the fluid and increases the fluid pressure. The diffuser also serves to minimise the whirl velocity of the fluid at the diffuser outlet, and provides for substantially axial fluid flow from the stage. Due to the whirl imparted to the fluid by the impeller, separation of the gas and liquid phases towards the hub of both impeller and diffuser within the pump stages limits the gas-handling capability of such pumps. Accordingly, it is among the objects of another aspect of the present invention to provide a pump for multiphase fluids in which the gas phase remains substantially entrained within the liquid phase, thereby enhancing the ability of the pump to handle large gas fractions in the total fluid flow.